The present invention relates to a method and apparatus for detecting registration mark on a wafer for alignment, and more particularly to a method and apparatus for reduction-projection position detection on a TTL (Through The Lens) system for use in a laser exposure apparatus using excimer laser beam of narrow wavelength band.
A lens for a projection exposure apparatus is difficult to be compensated for chromatic aberration and the lens has hitherto been designed and put to practical use on the assumption that the exposure is made using single wavelength light. Therefore, light of single wavelength must have been used also for illumination in detecting the registration mark. A state of illumination and detection in action will be described with reference to FIG. 13. Single-wavelength illuminating light 2 is projected from above a mask 1 to illuminate, through a reduction-projection lens 3, a registration mark 5 on a wafer 4. Reflected light therefrom is reflected by a half mirror 6 and detected by a detector 7. FIG. 13 shows an image detected by the detector 7, wherein the registration mark 5 is seen within a window 8 in the mask. As apparent from the enlarged view of the registration mark, resist 9, which is required for pattern formation, is spread over the registration mark.
The cross section of the part in question, as shown in FIG. 14, can be studied after being imaginarily separated into a mark portion 11 and a resist portion 12. The light of single wavelength illuminating the resist portion from above causes interference therein. The interfered light will be uniform if the resist has an even thickness. In practice, however, there is present unevenness on the resist due to unevenness on the surface of the mark, and therefore, the interfered light therein becomes irregular. Thus, there has been a problem that the accuracy in the detection of the mark is thereby lowered.
To solve this problem, an art disclosed in Japanese Laid-open patent publication No. 60-80223 used light of two wavelengths selected from emission lines (lines of remarkably bright light) included in light from a mercury lamp, i.e., h-line (of 405 nm wavelength), g-line (of 436 nm wavelength), e-line (of 546 nm wavelength), and d-line (of 577 nm) as the illuminating light, and reduced the effect of the unevenness on the resist by summing up then obtained results. Detection signals obtained when the e-line and d-line were used for the illumination and the summed up signal thereof are shown as an example in FIG. 15, from which it is known that both ends 13 of the mark 5 were symmetrically detected by summing up the two signals and the effect of the unevenness of the resist was reduced.
In place of the aforesaid method, there was disclosed another method in Japanese Laid-open Pat. No. 60-136312 using seemingly different wavelengths of light for the illumination and summing up the detected signals. In this method, as shown in FIG. 16, a laser beams 14 having a high rectilinear propagating property was used as the illuminating light, and a reflecting mirror 15, which was inserted in the way of illumination, was caused to swing so that the illuminating light was swung about the registration mark 5 being taken as the center of the swinging. This method utilized the change in the optical path length within the resist 9 produced by the swinging as shown in FIG. 17.
The excimer laser used in the present invention is a narrow band laser beam of 249 nm wavelength (other than this, there are those of 193 nm, 308 nm, and 351 nm wavelengths). The reduction-projection lens applied to it is not at all compensated for chromatic aberration, and therefore, it can be used only for a laser beam. The quantity of chromatic aberration in general is extremely great. The image forming position 21 in FIG. 16 is caused to shift about 4 nm by 1 nm change in the wavelength. Since the band of the emission lines (h-, g-, e-, and d-lines) of the light from the mercury lamp is 1 to 2 nm thick or above, the information forming position shifts as large as 4 to 8 nm, and therefore, they cannot be used for detection of the registration mark.
Then, even if the emission lines from the mercury lamp were used, the range within which the effect of the uneven thickness of the resist is kept low is from 0.9 to 1.4 .mu.m as described above with reference to FIG. 15, and so, in the case the resist thickness is outside the range, the reflected light will show a great periodical variation. While the case where e-line and d-line were used for the illumination was shown in FIG. 15, other cases, that is, g-line+h-line, g-line+e-line, h-line+d-line, and g-line+h-line+e-line were used as shown in FIG. 7. It shows that only the use of g-line+h-line was effective against change in the thickness from 0.7 to 1.0 .mu.m, but the rest were not at all effective. Various other combinations were tried but found to be ineffective at all.
As described above, the prior art cannot be applied to the reduction-projection lens designed for excimer laser beam, and has no sufficient effect against the change in the thickness of the resist.